Brillouin Scattering with Neutrons in an Amorphous Solid

The dispersion of collective models in the amorphous solid, Mg 70 Zn 30 , has been determined down to momentum transfers below the first pseudo-Brillouin zone for the first time using epithermal neutrons at the high-resolution time-of-flight spectrometer HET at the spallation source ISIS. In the region of Q values, where the previous experiment done with thermal neutrons and the present experiment overlap, the two sets of results agree very well. Also the previous discrepancies with the dispersion determined from the calculated one-phonon spectral function for the same metallic glass reported earlier are now less severe in the comparison with a more recent and more complete theoretical investigation of the dynamics of this glass. The extension into the first pseudo-Brillouin zone by neutron Brillouin scattering now allows this dispersion branch to be assigned to predominantly longitudinal excitations. Previously, it was erroneously assumed to be a transverse branch due to the discrepancies with the earlier theoretical results.